Nonlinear runup resonance in a finite bay of variable parabolic cross-section Nazmi Postacioglu and M. Sinan Özeren Istanbul Technical University Abstract Matching with radiation During the recent years, several interesting studies published on the non-linear runup of Tsunamis and other incident waves in channels and bays. Most of these studies use Riemann invariants to tackle the nonlinearities associate with both the incident and reflected phases. However, all of these studies assume that the waves get generated within the bay (essentially assuming bays extending into infinity), rather than conisdering a wave that has been generated in the open sea. Such waves would be reflected not just by the shoreline but also by the bay mouth. This is a setting where a resonance can be induced within the bay. We investigate this problem with a modal approach. If we now make a cylindrical approxiamtion at the mouth of the channel, then we obtain Now let us seek a solution of the form then satisfies The points are the singular points of this ODE, however for a set of discrete values there are regular solutions at both singular points. Now let us try to write the differential opeator that corresponds to the above ODE in terms of a matrix using a spectral basis. To do this, let us start with the part of that operator that does not involve , this leads us to which is a diagonal matrix. Here are Legendre polynomial polynomial of order 2i. Note that if is small enough there is only one negative element on the the diagonal of matrix A Define also symmetrical positive tridiagonal matrix for the other part of the differential operator as so the values are the eigenvalues corresponding to the generalized eigenvalue problem given as Now, if the wavelenght of the incident wave is larger than , then only one of the eigenvalues will be negative , in other words The proposed solution is then where are the elements of the eigenvector matrix corresponding to the generalized eigenvalue problem. The phase factor will be found in a way to insure a smooth transition to the solution found by Didenkulova and Pelinovsky (2011) . Their solution in the linear limit reads The first term in our solution and this solution abover by Didenkulova and Pelinovsky have the same logarithmic derivative at which reads To macth the free surface at the channel mouth, we have the following solutions for the outer side and the flux associated with this is then where the coefficients are calculated so as to minimize the following penalty integral Reference [1] Didenkulova I, Pelinovski E. Phys. of fluids 23 086602 (2011) Background Fig. 1. Problem geometry, a sloping bay with parabolic cross-section Let us start our analysis by writing down the shallow water equation for two spatial directions (x and y) where and is the ratio between width and depth. Now if we define we obtain where .